The present invention is directed generally to infrared optical instruments such as cameras and spectrometers, and particularly to such cameras and spectrometers as are able to obtain data at a high repetition rate. Infrared cameras and spectrometers capable of repeatedly obtaining data many times per second are desirable to provide detailed information in dynamic thermal situations such as occur in fires and high temperature manufacturing and processing operations.
Obtaining accurate information on the structure of fires is vital in a wide variety of situations. These include the detection of pollutant emissions and process variations requiring control in industrial applications as well as fire detection and safety situations. The structure of fires can be determined experimentally by measuring the gas velocity, gas species concentrations and temperatures in non-luminous flames. In luminous flames, additional information on the soot volume fraction or particulate concentrations is also important. In turbulent and transient flames, these measurements are preferably made simultaneously so that any cross-correlation between them can be computed as well.
Most combustion processes, whether accidental or in industrial burners, gas turbine engines or other machines that use hydrocarbon fuels, emit H2O and CO2 as the major byproducts of combustion. In order to estimate flame or hot gas temperature, as well as gas species concentrations in reacting flows, it is critical to obtain simultaneously emission or absorption spectra from the fundamental H2O (2.7 xcexcm) and CO2 (4.4 xcexcm) bands. In addition, most accidental and industrial flows are turbulent. Therefore, it is an object of the present invention to obtain the desired thermal spectra over very short periods of time to prevent blurring of the information. It is additionally desirable to provide a dynamic time-based set of data by obtaining the spectra repeatedly at a high interval rate reflecting the short period of time required to eliminate data blurring.
Likewise, many industrial processes involve the thermal treatment of flows of materials although not involving flames or combustion. The uniformity of the products of these industrial processes is often achieved through close control of the thermal characteristics. Thus, it is often desirable to continuously measure the thermal characteristics of a flow of materials, such as continuously extruded polymer films, to detect any thermal change that might portend an undesirable modification in the product. It is often desirable to measure the absorption of thermal energy over a large number of wavelengths at a sufficiently fast rate so as to enable on-line monitoring of a process, for example, in food and chemical processing lines. An object of the present invention is to have the thermal measurements preferably taken as often as possible over the smallest physical region possible so as to isolate any area of possible product change. At the same time, it is desirable to assay the entire product flow, not just discrete portions thereof, so that any changes in thermal characteristics can be identified as soon as they occur with some precision as to location. The thermal measurements are most desirably made by a thermal imaging camera having a data output that could possibly be used as an input for a closed loop process control.
The general design of cameras and spectrometers of the present invention that allow the above design objectives to be met are preferably constructed so as to be minimally affected by the vibration commonly present in industrial situations. The cameras and spectrometers of the present invention are preferably constructed using principally off-the-shelf elements that are competitively priced to maintain the cost of the overall instrument at a reasonable level.
The use of charge storing photon integration devices to form one and two-dimensional opto-electric arrays for converting infrared radiation into a suitable electronic signal is known, for example, from U.S. Pat. No. 5,166,755. The arrays generally consist of a specific arrangement of closely adjacent photosensitive sites, elements, or pixels situated on a monolithic substrate and directly coupled to electronic elements that permit interrogation of the array by scanning or polling. The information gathered by the interrogation can thereafter be processed and/or stored for contemporaneous or future readout or display. The arrays specifically take the form of CCDs, CIDs, DDPDs, SSPDs, or the like which include an integrated circuit which contains the electronics for sequentially scanning and reading the signal of each pixel in the array in a designated order. The electronic circuitry can also include elements for calibration, pixel spectral response correction, pixel uniformity corrections, background signal suppression, dark current suppression, time delay integration, and various other signal enhancements. The photosensitive pixels are disclosed to be constructed using materials specifically selected for infrared sensitivity such as PtSi, HgCdTe, or InSb, which are also disclosed to be used in U.S. Pat. No. 5,420,681. In some situations, electronic gating is employed in the place of a mechanical shutter to control the output signal in relation to the optical input. The arrays are specifically disclosed in relation to their use in infrared spectrometers, but other uses for similar devices are suggested such as in thermal imaging cameras. The systems are, however, unable to provide data over the whole range of from 1.2 to 5 xcexcm at temperatures ranging from 210 to 293xc2x0 K and require cooling with cryogenic liquids such as N2 which is undesirable, particularly in the case of portable equipment.
The use of a sixty-four element PbS detector array in an infrared spectrometer is disclosed in U.S. Pat. No. 5,394,237. The array is coupled to a multiplexer which sequentially samples the signal level from each detector in the array to arrive at an output which is fed to an amplifier and then to an analog/digital converter. The digital output from the converter is controlled by a decoder coupled to receive signals from and send output signals to a standard parallel port of a personal computer for data storage, display, and/or processing. The spectrometer also includes a chopper-type shutter located at the entrance slit and controlled by control circuitry to open for a programmed number of read cycles of the decoder and to then close for another programmed number of read cycles. The output obtained during the closed shutter condition is employed to provide a dark current baseline measurement for correction of the light current measured signal. The entire spectra can be sampled at a rate of 80 Hz using this instrument. This instrument avoids the need to use liquid cryogenic cooling by adopting the more sensitive PbS array that is sensitive over the range of from 1.0 xcexcm to 3.0 xcexcm. However, the instrument as a whole fails to have a satisfactory useful spectral range due to the use of a grating that causes multiples of any given wavelength, e.g. 1.2 xcexcm and 2.4 xcexcm, to be dispersed to the same location on the array. Thus, the instrument is usefully functional only over a range of either from 1.0 xcexcm to 2.0 xcexcm, or from 1.5 xcexcm to 3.0 xcexcm. The upper limit of sensitivity of 3.0 xcexcm also fails to include the hot CO2 (4.4 xcexcm) band which is very desirable in any analysis of combustion or other processes where hot CO2 might be present. The sampling rate of 80 Hz is also less than desirable for use in high-speed industrial processes.
A near infrared analyzer employing a PbS array containing 256 pixels is disclosed in U.S. Pat. No. 5,422,483 for use in the range of from 1.0 xcexcm to 2.6 xcexcm. Alternatives for the PbS array are indicated to be made of InGaAs, InGaAsP, or PtSi. The array is used with a diffraction grating to analyze the infrared signature of, for example, processed foods for protein and water content. This analyzer omits not only the hot CO2 (4.4 xcexcm) band which is very desirable in any analysis of combustion, it also omits the fundamental H2O (2.7 xcexcm) band as well. The output from the array is to a preamplifier and integrator. The integrated signals are sequentially retrieved by a multiplexer and delivered through an A/D converter to a central processing unit of a personal computer (PC-CPU). There is no specific disclosure of the protocol employed for transfer of the data to the PC-CPU. It is likely that a standard serial or parallel port using RS122 was employed much like that disclosed in U.S. Pat. No. 5,394,237, which significantly limits the data transfer rate from the analyzer to the PC-CPU.
An infrared detector adapted to capture data at a high repetition rate includes a base with an aperture-defining element fixed to the base for periodically admitting infrared radiation from a source. An at least one dimensional array of pixels is fixed to the base so as to be optically aligned with the elongated source to intercept the periodically admitted infrared radiation for generating a set of data indicative of the infrared characteristics of the source. A data collection element is coupled to the pixel array and to the aperture defining element for collecting the set of data a selected number of times during each opening of the aperture defining element. A serial output on the data collection element provides a list of data values representative of the infrared intensity at each pixel following each collection of data which is transferred to a resident memory by means of a universal serial bus which allows for very rapid data acquisition at rates up to 6250 Hz.
Preferably, the base forms a portion of an enclosure. The radiation from the source enters the enclosure through an input port and is incident upon an entrance slit that provides the desired spatial resolution of the infrared radiation source. The aperture-defining element includes a chopper, such as a tuning fork chopper, driven at a defined frequency of between about 300 Hz and 25 kHz. If the source is large and located very near the slit, an additional imaging element is not required to image the source onto the slit. A plurality of optical components then direct the infrared radiation from the aperture defining element to the at least one dimensional array of pixels. A plurality of fixed mounts are provided for mounting each optical component to the base at a single pre-selected fixed position that defines the optical alignment between the source and the array of pixels and thereby minimizing the influence of environmental vibration.
The optical elements can include lenses and mirrors, and in the case of a spectrometer, also includes at least one spectrum splitting element fixed to the base for dividing the admitted infrared radiation into spectral components that are displayed across the array of pixels. As the wavelength varies across the desired spectrum, the slit image shifts along the detector array. In a thermal imaging camera according to the present invention, the thermal measurements are most desirably made by the optical elements aligning a plurality of regions of the elongated source with various regions of the array of pixels.
The spectrum splitting element employed in a spectrometer of the present invention preferably takes the form of at least one prism composed of a material selected from the group consisting of calcium fluoride and lithium fluoride, although a diffraction grating could be employed with significantly diminished spectral scope. The preferred instrument uses a pair of equilateral calcium fluoride dispersing prisms mounted in close proximity to each other which avoids the multiple wavelength problem common to all diffraction grating systems.
The pixel array is preferably composed of PbSe since such an array is responsive to radiation over the range of from 1.2 xcexcm to 5.0 xcexcm thereby including both the hot CO2 (4.4 xcexcm) band and the fundamental H2O (2.7 xcexcm) band. The PbSe pixel array can be combined with a diffraction grating adapted to detect radiation having a wavelength of at least between about 2.5 and 4.6 xcexcm without experiencing the wavelength multiple problem previously identified yet still including the most desirable data bands. The preferred array includes 160 pixels arranged as two adjacent parallel rows that are 4 mm in length. The pixels of the preferred array are divided into two banks of 80 pixels corresponding to the two rows and referred to as the odd and even banks. The image of the slit is preferably over two pixels wide and is dispersed lengthwise over less that the entire length of the array to compensate for any error in alignment. In the preferred embodiment the slit image occupies about 90% of the pixel row length.
The pixel array provides an analog current output that is proportional to the change in light energy falling on each of the elements. This change in intensity is created by the motion of the chopper past the slit. The current from each element in a bank is multiplexed onto the output by a clock input at a fixed frequency, preferably at about 1 MHz. The multiplexed current is converted into voltages using a biasing circuit and then amplified. The digital and analog components required for the pixel array, the chopper, the multiplexer and the amplifier are provided by a drive circuit.
The amplified signal from the array is sampled when the chopper is opened and closed to provide a plurality of voltages from the entire array. The differences in the voltages with the chopper opened and closed respectively, provide a direct measure of the intensity of light incident on the pixel elements of the array. The voltage differences are digitized by an analog to digital converter and the data collected can then be directly transferred to a suitable computer memory at a rate reflective to the data acquisition rate. The output of data to the computer memory is via a universal serial bus.
One distinct feature of the present invention is the use of a unitary base and a plurality of fixed mounts provided for mounting each optical component to the base at a single pre-selected fixed position that defines the optical alignment between the source and the pixel array. This feature has the advantage of minimizing the influence of environmental vibration on the optical instrument.
Another distinct feature of the present invention is the alignment of the image of the slit so that it is dispersed lengthwise over less than the entire length of the array, while the slit image width in the cross-dispersion direction is greater than the width of the array. The feature has the advantage of providing an automatic compensation for small errors in alignment. In the preferred embodiment the slit image occupies about 90% of the pixel row length.
Yet another distinct feature of the present invention is the use of a PbSe array which ensures the collection of data over a range including both the hot CO2 (4.4 xcexcm) band and the fundamental H2O (2.7 xcexcm) band.
Still another distinct feature of the present invention is the use of one or more prisms composed of a material selected from the group consisting of calcium fluoride and lithium fluoride, which avoids the multiple wavelength problem common to all diffraction grating systems.
An additional distinct feature of the present invention is the use of a universal serial bus for data transfer from the A/D data converter at the output of the array to the memory which allows for much higher data acquisition rates than can possibly be accomplished using a standard serial or parallel port using RS122 or similar protocol.
Other features and advantageous of the present invention will become apparent to those skilled in the art from a consideration of the following description of preferred embodiments of the invention. The description has reference to the accompanying drawings that illustrate the best mode of the invention.